In display applications, optical clear adhesives (OCA) or liquid optical clear adhesives (LOCA) are usually used to bond a touch panel or cover glass to a substrate which could be distortion sensitive. Optical clear adhesives used in the area of HHDDs will have potential problem of light leaking after long term use, especially for big displays after long-term temperature/humidity change. The root cause is the build-up of the stress on the distortion sensitive substrate and on the adhesive. The stress will result in the change of the orientation of the molecules in the substrate. As a result, the path of the light will be changed, and the light from the device will be guided to the edges, and in turn the quality of the images will be reduced.
Many efforts have been made to provide LOCAs which can significantly decrease or eliminate the build-up of the stress and avoid the light leaking.
For example, US 20130271828 A1 discloses an optical bonding layer including an optical film e.g. pressure sensitive adhesive (PSA) film and a first liquid optically clear adhesive (LOCA) positioned adjacent the optical film. The LOCA was used to relieve the stress on the PSA and on the polarizer. However, the process becomes more complicated, since two steps of the application of both PSA and LOCA are required. In addition, the LOCA was cured instantly, and the stress from the PSA during the paving and the stress from the LOCA during the curing will be added up.
U.S. Pat. No. 5,795,650 A discloses a pressure-sensitive adhesive sheet which comprises a base material layer; and a pressure-sensitive adhesive layer which contains a pressure-sensitive adhesive composition and a plasticizer, wherein the weight-average molecular weight of the main polymer of the pressure-sensitive adhesive composition of the pressure-sensitive adhesive layer is 600,000 to 2,000,000. The plasticizer is supposed to be effective in relaxing a stress due to the polarizer shrinkage. However, using a PSA with the capacity for stress relaxation to relieve the stress build-up in a polarizer unit shows insufficient durability in resisting the formation and growth of bubbles and edge lifting in high temperature and high humidity conditions.
U.S. Pat. No. 6,800,366 B2 discloses an acryl-based PSA composition comprising a component having a positive stress optical coefficient. The purpose of introducing the component was to compensate the birefringence caused by the stress in the polarizer and PSA. However, the control on the compensation cannot be precise due to the complicated change in the structures of the polarizer and the PSA during the thermal cycles. In addition, the light leaking cannot be avoided.
As such, there remains a need to develop an improved adhesive composition which can solve the problem of the build-up of the stress and light leaking, while remaining other performances such as high transmittance and high stability under high temperature/humidity.